A hot film bridge type device for measuring flow velocity is described in U.S. Pat. No. 3,996,799. That device employs a chip on which two elongate temperature sensitive hot film resistors are situated with their longitudinal axes forming a 90.degree. angle. Two other resistors which are also situated on the chip form a bridge with the two temperature sensitive resistors. The bridge is energized in the conventional manner by connecting two opposed junctions of the bridge arms to a source of electrical power. The other two opposed junctions of the bridge arms are connected to the input of a differential amplifier to provide the measurement signal.
As stated in U.S. Pat. No. 3,996,799 in column 2, lines 47 to 55, in most instances four perfectly identical resistors cannot be made on the chip and, consequently it is necessary for precise measurement, to provide compensation for changes in ambient temperature because all four resistors do not respond identically to temperature changes. That U.S. patent describes an arrangement for providing compensation for ambient temperature changes by deriving a temperature dependent signal from one of the resistors and adding it as a compensating signal to the measurement signal. In that compensation arrangement, a second differential amplifier obtains its input from one of the bridge resistors and the outputs of both differential amplifier are summed by a third amplifier.
The ambient temperature compensation arrangment of U.S. Pat. No. 3,996,799 works satisfactorily in most situations but it has been found that that arrangement does not provide full compensation for ambient temperature changes where the flow velocity to be measured is very low.